The present invention relates to a closed cycle refrigeration system for use in a refrigerated display case. Means for maintaining a high head pressure to permit gas defrost are included and a simplified and low cost gas defrost feature for the evaporator coils located in the case is provided to improve efficiency of operation.
In the basic construction and operation of a closed cycle refrigeration system, gaseous refrigerant, e.g., freon, is compressed to a high temperature and pressure. The compressed gas is passed to a condenser where it is condensed to a liquid phase. The pressure within the condenser is maintained high enough that the condensing temperature is higher than the ambient air temperature. The liquid refrigerant may be temporarily stored in a receiver before being passed, through a metering device to reduce the liquid refrigerant pressure, to an evaporator located within a display case. As the liquid passes through the evaporator, it extracts heat from the display case and undergoes a phase change to the gaseous state. This low pressure gaseous refrigerant is supplied to the input side of the compressor where it is heated and compressed to a high pressure and the cycle is continued.
Traditionally, the condenser was operated at a preselected design temperature level. The design temperature for the condenser was generally determined as a function of the highest ambient temperature during a normal period of the warmest season in a particular area. The condenser was operated so as to condense the gaseous refrigerant at a temperature of at least 10.degree. F. above this design temperature. Consequently, if the design temperature was 90.degree. F., then the condenser temperature was set at 100.degree. F.
Recognizing that the design temperature was only likely to occur a few days in a year, and then only during the day and not at night, the refrigeration systems have been modified so that the condenser temperature followed the path of the ambient temperature while always remaining at least 10.degree. F. above the ambient temperature. Varying the condenser temperature to follow ambient conditions results in increased compressor capacity. The rule of thumb is that every 10.degree. F. drop in the condenser temperature increases the compressor capacity by about 6%. Thus, if the condenser temperature drops from 100.degree. to 75.degree., the compressor capacity will increase by about 15%. Simultaneously, the compressor consumption will be reduced, the compressor efficiency will increase, and the BTU/Watt of the compressor will increase. The combination effect is to increase compressor capacity and reduce power consumption, so that for every 10.degree. F. drop in the condenser temperature, there will be approximately an 8% reduction in power consumption, assuming constant refrigeration load.
Another need in refrigeration systems is to provide for defrosting of the evaporator coils. A defrost cycle for this purpose can be actuated either at set periodic time intervals or when the defrost build-up within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. In this manner of operation it is possible to avoid any significant frost build-up within the evaporator coils such that inoperability of the refrigeration system would occur.
There have been three different approaches for defrosting refrigerated display cabinets in this art. These are, utilizing electric resistance heaters; passing a compressed refrigerant gas having a high specific heat through the refrigeration coils; and, circulating ambient air through an air conduit in which the refrigeration coils are positioned. In order to utilize a compressed refrigerant gas as a source for energy to be employed during a defrosting cycle it is necessary to construct the entire refrigeration system in a manner which permits the low energy-consumption functioning of the system in the manner above-described as well as to balance the pressures and temperatures at various points in the system so as to achieve an efficient gas defrost operation under a wide range of ambient conditions which are encountered in the locations in which such systems are used.
This invention relates to a refrigeration system for use in cooled display cases of the type which are used primarily in retail food and supermarket outlets in which a defrost feature is included for defrosting evaporator coils in an operative and thermally efficient manner.
During the operation of the refrigeration systems, it is necessary to regulate the pressure within the receiver in order to ensure proper operation of the evaporators. Such regulation has typically been provided by shunting hot gaseous refrigerant from the gas discharge line of the compressor directly into the receiver whenever the relative pressure of the receiver drops by more than a preselected pressure differential from the pressure in the gas discharge conduit. For such purposes, a check valve, typically set to respond to a pressure difference on the order of 20 or 30 psi as compared to the pressure in the gas discharge conduit the check valve opens and allows the hot gas from the gas discharge line to flow directly into the receiver. Since the gaseous refrigerant in the gas discharge conduit is typically of a temperature level of approximately 200.degree. F., such gas acts as a significant heat source to the receiver, a situation which is generally undesirable.
During the refrigeration cycle, the refrigerant absorbs a substantial amount of heat during the evaporation stage, which heat is then dissipated by the condenser as a waste by-product of the refrigeration cycle. A technique for taking advantage of the heat to be dissipated by the hot gaseous refrigerant is the utilization of a heat recovery coil, such as shown in U.S. Pat. No. 4,123,914 issued Nov. 7, 1978, to Arthur Perez and Edward Bowman, and commonly assigned with the present invention. The disclosure of the Perez et al '914 patent is incorporated herein by reference. Such a heat recovery coil allows for extraction of heat from the gaseous refrigerant flowing out of the compressor before entering the remote condenser. Such extracted heat then can be utilized for heating the interior of the building where the refrigeration system is employed.
Especially in recent years, much attention has been directed to improving the efficiency of such refrigeration systems. The prior art is replete with discussions of various techniques for attempting to improve the operation of a refrigeration system. In large installations, such as supermarkets, there are typically large numbers of refrigerated display cases and a plurality of compressors are used to satisfy the heavy refrigeration load under certain conditions, such as during the warmer portions of the year. The efficiency of the compressors is dependent upon the compression ratio of the discharge side of the compressor to the suction side of the compressor. Thus, by reducing the head pressure at the compressor discharge, the efficiency of operation of the compressor can be increased. One such system, employing reduced head pressure to increase operating efficiency, is described in co-pending application Ser. No. 57,350, filed July 13, 1979, titled ENERGY SAVING REFRIGERATION SYSTEM, now U.S. Pat. No. 4,286,437, and commonly assigned with the present invention; the disclosure of said Ser. No. 57,350 is hereby incorporated by reference as though fully set forth herein.
One of the features of the low head pressure systems, particularly including the one described in the aforesaid application Ser. No. 57,350, is that the system is designed to subcool liquid refrigerant in the remote condenser. Liquid subcooling will increase the efficiency of the system since the refrigerant will extract 15-25% more heat per pound circulated. The rule of thumb is that for every 10.degree. F. subcooling the system efficiency will increase by 5%. In substantially all commercial refrigeration systems, a receiver tank or surge tank is interposed between the condenser output and the liquid manifold supplying the evaporator coil. It has been found that, in systems employing a receiver tank, the refrigeration loses 10.degree. to 15.degree. F. of subcooling in passing through the receiver; that is, the temperature of the refrigerant in the receiver may rise 10.degree. to 15.degree. F. This results in a loss of efficiency since fewer BTU's of heat can be extracted from the air around the evaporator coils in the display case for each pound of refrigerant passing through the evaporator coil. One reason for this heat gain is that the receiver tank is generally located in the machinery room adjacent the compressor motors and related heat producing equipment. Some of this heat will be absorbed by the refrigerant in the receiver and the temperature of the refrigerant will rise accordingly.
Some commercial refrigerating systems attempt to avoid the problem of receiver tank heat gain by using a surge tank; one such surge tank system is shown in U.S. Pat. No. 3,905,202 issued Sept. 16, 1975 to Donald F. Taft et al. In a surge tank type of system, condensed liquid refrigerant flows directly from the condenser output to the case evaporators. The surge tank stores excess liquid refrigerant to assure continued operation under varying ambient conditions which result in a variation in the condensing capacity of the condensers. It has been found that, especially during hot weather operations, the closed circuit system may "die" because the surge tank pressure may run 35 to 40 psig lower than the condenser pressure, resulting in liquid refrigerant logging in the receiver and not being passed to the evaporator. This problem is particularly prone to occur during periods of abnormally high ambient temperature; at such times, the pressure in the condenser will correspond to an ambient temperature of 90.degree. F. to 100.degree. F., for example, whereas the surge tank temperature and thus pressure will be lower so that the refrigerant liquid will flow into the surge tank. The liquid thus tends to flow into the surge tank and create a logged condition which deprives the evaporators of refrigeration capacity during high ambient temperature conditions.
One design for providing hot gas defrost for a refrigeration system is shown in U.S. Pat. No. 4,012,921 to Willitts et al. In this patent a defrosting line is connected into a hot gas discharge line from the compressor. A series of defrost system valves are provided for controlling the flow of hot gas through the defrosting lines. The compressor discharge line has a pressure regulated valve therein which is responsive to the receiver pressure in order to maintain the defrost line pressure above the receiver pressure during both refrigeration and defrost cycles. In conditions where this valve remains open, e.g. when an adequate receiver pressure is being maintained, the system does not modulate the defrost gas pressure at the beginning of a defrost cycle. This results in a delay in the defrost cycle time under ambient conditions such as low temperature during which low head pressure can exist in the defrost gas line. Another problem is that this refrigeration system design does not protect against the system "dying" due to logging of the receiver during periods of abnormally high ambient temperature such as referred to above. Under this condition an inadequate flow of refrigerant in order to achieve specified levels of refrigeration in the evaporator coils can exist during those times periods when the refrigeration load is the highest. The operation of the main defrost gas valve dependent upon the receiver pressure would accentuate this type of problem during the refrigeration cycle functioning of the evaporator coils.
Another refrigeration system provided with hot gas defrost, but not having the increased efficiency features of the present invention is described in U.S. Pat. No. 4,276,755.
The present invention constitutes an improvement over prior art receiver tank and surge tank systems having provision for hot gas defrost. The present invention incorporates a hot gas defrost system which is responsively controlled by the initiation of a defrost cycle so that a high head pressure is immediately obtained in order to insure that the defrost gas will pass through selected evaporator(s) in the reverse direction from the refrigerant flow during the normal refrigeration cycle.
The improved system can also incorporate a by-pass conduit which permits subcooled liquid refrigerant to flow directly from the condenser to the evaporator coils under normal temperature conditions without first passing through the receiver tank. This arrangement will obtain a complete liquid flow in the conduit supplying the evaporator coils. Receiver designs with a single bottom-connecting conduit can result in a mixture of liquid and gas in the evaporator conduit because at high refrigeration loads the liquid is drained immediately from the condenser.
In one embodiment of the present invention, the receiver tank is configurated to have its input and output located at the bottom of the tank. The lower portion of the tank is insulated to minimize heat transfer from the machine room to the liquid refrigerant in the bottom portion of the receiver tank. Minimization of heat transfer to the liquid refrigerant is of important in order to maintain the subcooling condition achieved in the condenser.